Biomechanics – Stability & Balance (A review)

Centre of Gravity (COG) The point in the body about which all parts are in balance – or – the point through which gravity acts upon the whole body. Can lie inside or outside the body Can be raised or lowered by moving parts of the body up or down

Centre of Gravity Centre of gravity can be defined as “the point at which all parts of the body are equally balanced”. Because the centre of mass is the point of an object where all parts are evenly balanced, it changes depending upon what position the body is in.

Centre of gravity can lie inside / outside the body CoG lies around the navel in neutral upright position with arms at side

CoG can lie outside the body: E. g CoG can lie outside the body: E.g. in dive roll / tuck, pike, fosbury flop

Base of support (BOS) The area bordered by the parts of the body in contact with the surface

Base of Support Base of support is the area within an object or person’s point of contact with the ground. The larger / wider the base of support = more stable Narrow base of support = less stable

Line of Gravity (LOG) The line running vertically from COG to the centre of the earth May or may not pass through the BOS (i.e. LOG still passes through a person/object when the person or object is in flight or moving forward or changing direction)

Line of gravity and base of support and physical activity Top of body moves towards line of gravity Direction of movement Leg pushes against ground Base of support

7 Key Principles Athletes can increase their stability if they extend their BOS in the direction in which the force is being applied By increasing the size of the BOS as the athlete moves in the direction the force is applied, the LOG stays inside their BOS maintaining stability E.g. Pitcher in softball taking large stride (student supplies example)

Stability is improved through: Widening the base of support

7 Key Principles Athletes can increase their stability when the LOG falls within the margins of the BOS When the COG is inside the margins of the BOS, stability is maintained. The closer the COG/LOG gets to the edge of the BOS, the greater the chance of losing stability E.g. Gymnast on beam (student supplies example)

7 Key Principles Athletes can increase their stability if they lower their COG with respect to the BOS When the COG is lowered it has a greater distance to travel before reaching the margins of the BOS E.g. Scrummaging (student supplies example)

Stability is improved through: Lowering the centre of gravity

Athletes can increase their stability by increasing their mass 7 Key Principles Athletes can increase their stability by increasing their mass An increase in mass means an increase in inertia which makes moving the COG/LOG outside the margins of BOS more difficult E.g. Size and bulk of sumo wrestler (student supplies example)

7 Key Principles Athletes can increase their stability if they move their LOG towards an incoming force If the athlete moves the COG in the direction that force is applied, once they are contacted, the COG moves back through the BOS which helps keep them stable E.g. Collision situation in rugby (student supplies example)

7 Key Principles Athletes can increase their stability if they increase the size of their BOS A broader BOS means there is more space for the COG and LOG to travel before reaching the edges of the BOS E.g. wide stance in basketball defence (student supplies example)

7 Key Principles Athletes can increase their stability if friction between surfaces is increased An increase in friction means it is harder to move the COG outside the BOS E.g. Sprigs / spikes (student supplies example)

Skiing Skiers wax their skis to decrease friction to make movement over the snow easier.